Spatial variation in the kinetics of aluminium (Al) uptake in roots of wheat (Triticum aestivum L.) exhibiting differential resistance to Al.: Evidence for metabolism-dependent exclusion of Al

We have investigated the kinetics of-Al uptake into various root segments and have compared uptake between two wheat (Triticum aestivum L.) cultivars that differ in their ability to resist Al toxicity. Following 60 and 180 min of exposure to Al, we found no differences in total Al accumulation between root tips (0-5 mm) and more mature root regions (5-10 and 10-20 mm). However, when roots were exposed to Al and subsequently desorbed in citric acid to remove exchangeable Al in the cell wall and in NH4Cl to remove the mucilage, non-exchangeable Al levels in root tips were 33 to 37% greater than in mature root regions. When the rate of accumulation of non-exchangeable Al in 20 mm root tips between an Al-resistant cultivar (PT 741) and an Al-sensitive cultivar (Neepawa) was compared, no differences were observed. When the same comparison was made in 5 mm root tips, the rate of Al accumulation in the AL-sensitive cultivar was nearly twice that in the Al-resistant cultivar. Furthermore, the rate of Al uptake in the Al-resistant cultivar increased dramatically in the presence of the metabolic inhibitor and protonophore, DNP, while DNP had little effect on uptake in the sensitive cultivar. This effect of-DNP was also specific to root tip tissue. Our results support the hypothesis that DNP inhibits an AL exclusion mechanism operating at the plasma membrane in Al-resistant cultivars.